During recent years, olefin metathesis has seen extraordinary development and has turned out to be a very versatile and efficient tool in organic synthesis. Ruthenium-based catalysts have become the first-choice catalysts for organic synthesis. During recent years, a huge effort has been dedicated to further develop and improve the characteristics of the Grubbs-type ruthenium catalysts. In this context, in 1998, Grubbs and co-workers disclosed a class of ruthenium-based catalyst for olefin metathesis involving bidentate Schiff base ligands 3. The synthetic protocol for the bidentate Schiff base Ru catalysts disclosed by Grubbs and co-workers, however, is saddled with a serious drawback, namely the use of a thallium(I) as counter ion for salicylaldimines (See Scheme 1 shown in FIG. 1) (S. Chang, LeRoy Jones II, Ch. Wang, L. M. Henling, R. H. Grubbs; Organometallics 1998, 17, 3460-3465).
Further improvement of Grubbs protocol has been given by Bjørsvik and co-workers, where the complexation with ruthenium was conducted by reacting one salicylaldimine ligand equivalent with 1 equiv of Grubbs first-generation catalyst (1) in presence of a 0.5 equiv of Ag2CO3 used as base (G. Occhipinti, V. R. Jensen, H. R. Bjørsvik, J. Org. Chem. 2007, 72, 3561-564). The complexation utilising the ligands containing nitro group provided a yield in the range 78-96%, which was as good as or better than the previously disclosed protocol.
The reaction utilising the ligand that lacks the electron-withdrawing p-nitro group, however, provided only a tiny yield, <2%. Attempts to improve the outcome of this reaction by prolonged reaction time and elevated temperature did not afford any significant improvements (a yield of only 4% was measured). It is believed that the major reason for the unsuccessful complexation was the relatively weak base (silver carbonate) that is used for the deprotonation of the phenolic group.
Further efforts concerning the synthesis of NHC-salicylaldimine complexes concentrated on using the standard means for introducing the H2IMes ligand, i.e., in situ formation of the free carbene from the imidazolinium salt with potassium tert-butoxide and subsequent reaction with the phosphine bearing ruthenium complex 3. Attempts to prepare complex 4 by this method resulted, however, in decomposition of the ruthenium benzylidene starting material. Faced with this roadblock, Verpoort et al. have chosen to approach 4 by introducing salicylaldimine ligands to a complex already bearing the H2IMes ligand (B. Allaert, N. Dieltiens, N. Ledoux, C. Vercaemst, P. Van Der Voort, Ch. V. Stevens, A. Linden, F. Verpoort, J. Mol. Cat. A: 2006, 260, 221-226). Alkoxide and aryloxide ligands, as well as neutral donors, react readily with bis(pyridine) precursor 6. The pyridine ligands in 6 are far more labile than the phosphine ligand in 5 and are displaced easily by incoming Schiff base ligand, especially with the assistance of thallium ions. Accordingly, the thallium salts of salicylaldimine ligands 2 were prepared in situ with thallium ethoxide and reacted with 6 (See Scheme 2 shown in FIG. 2).